Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

With empty annotations, the description carries the transparency burden. It discloses that the operation returns a JSON object with results in a 'values' object, which is helpful. It does not explicitly state that the tool is read-only or lacks side effects, but the wording 'Returns the cost' strongly implies a non-mutating lookup, providing minimal but adequate behavioral guidance.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

The main purpose is front-loaded in the first sentence, with the return format noted at the end. The two sentences about customer purchase behavior are extra contextual detail that may not be strictly necessary for invoking the tool, making it slightly less concise than ideal but still efficient.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

The tool has no parameters and an output schema exists, so the description only needs to indicate the tool's purpose and key behavior. It clearly states the return object schema and the business context around FUP tokens, which is sufficient for a zero-parameter query tool. It could mention authentication or error cases, but these are less critical given the simplicity.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

The input schema has zero parameters, so there are no parameter details to document. Per the baseline for 0-parameter tools, this scores a 4; the description appropriately focuses on the return format instead of parameter semantics.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

The description opens with a specific verb and resource: 'Returns the cost of a FUP token when performing a top-up.' It also clarifies that FUP tokens are additional workflow tokens purchasable on any subscription including freemium, distinguishing it from sibling tools like paywall__purchase_add_on_units or paywall__customer_deduct_from_fup.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description provides clear context: customers may purchase additional workflow tokens on any subscription including freemium, and the tokens do not extend the subscription period. This indicates the tool is for querying cost when considering a top-up. However, it does not explicitly name alternative tools or state when not to use it, so it lacks explicit exclusions.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.